Field of the Invention
The present invention relates to a scale device for use in, for example, a metal machining machine tool, an industrial machine, a precise length-measuring or angle-measuring device, or the like, as well as to a positional-information generation method and a multiaxial stage device.
Description of the Related Art
Moving table and stage of a highly precise class, which are used in a machine tool and a precise measuring device, are supported by a linear guide that is oil-pressure operated or provided with bearing balls or rollers placed therein. In the case when a rotary servo-motor is used as a driving unit, the rotary force of the motor is converted to a directly driving propelling force by ball screws or the like, and transmitted to a moving table or stage. Thus, the moving table or stage moves the workpiece, measuring object, tool or measuring device in forward and rearward directions or rightward and leftward directions, as well as in height directions. As a measuring device for measuring the position of the moving table or stage, an optical scale device and a magnetic scale device have been used.
Moreover, in order to carry out a machining process with higher precision, a method has been taken in which movement errors of a machine have been preliminarily measured and an instruction value containing a correction portion of the errors is generated so that movement precision of the machine is improved with repeatable reproducibility.
However, in an actual machine tool, since the weight and moments at the time of movements of a workpiece become different for each of machining processes, or since a guide surface of a bed wears with time or foreign matters are pinched therein, reproducibility between movements of a machine at the time when a correction value is acquired and actual movements at the time of the actual machining process is not necessarily ensured.
The inventors of the present invention previously proposed a technique (for example, see Accuracy Enhancement of High Precision Machine Tools by 2D Holographic Scale System 2011 the proceedings of ASCENTi-CNC2011 Annual Meeting) in which by using a two-dimensional holographic scale with high precision, a correction value for an actual machine tool is acquired with resolving power in the order of sub-nanometer, and by correcting an instruction value, the movement precision can be remarkably improved in comparison with the case in which no correction is made.
That is, in the previously proposed technique, as shown in FIG. 16, a two-dimensional holographic scale 3 serving as a reference scale is put on an XY table 2 of a machine tool 1, and by installing a two-dimensional sensor 4 at a tip of a spindle capable of moving in the Z-axis direction, movements of the machine tool 1 are preliminarily measured by using the two-dimensional holographic scale 3, and by forming a movement error map in accordance with a sequence of processes as shown in a flow chart of FIG. 17 so as to correct an instruction value, movement errors in the machine can be reduced. It has been confirmed through experiments that by adopting this technique, errors of a machine can be reduced.
However, in the case of actually carrying out a machining process in a machine tool, it is not possible to put a two-dimensional holographic scale serving as a reference scale on the XY-table of the machine tool.
Therefore, inventors of the present invention have carried out examinations as to what degree of error is generated after a correction due to influences from cutting reaction forces at the time of a machining process, a deflection of a bed caused by own weight of the workpiece, and the like, and have also carried out calculations by using a deflection calculation model in a small-size machine shown in FIGS. 18(A) and 18(B).
One reason for selecting this structure is that calculations can be carried out, with a change in gravity applied to a workpiece being excluded, and for simplicity of experiments, a machine main body is regarded as a rigid body, and it is supposed that no deformation due to an applied weight and moment inertia is caused and that an error is caused by a deformation of a guide considered to be the lowest in rigidity.
Moreover, in this calculation model, suppose that a cutting reaction force in the X-direction is represented by Fx, a cutting reaction force in the Y-direction is represented by Fy, a cutting reaction force in the Z-direction is represented by Fz, gravity applied to the X-axis is represented by Wx, gravity applied to the Y-axis is represented by Wy, a reaction force from a guide lower surface is represented by R1, another reaction force from the guide lower surface is represented by R2, still another reaction force from the guide lower surface is represented by R3, and the other reaction force from the guide lower surface is represented by R4.
In a deflection calculation model in this small-size machine, from equilibrium condition between own weight and cutting reaction force as well as equilibrium condition among moments around a roller, the following equations are satisfied:R1+R2+R3+R4=Wx+Wy+Fy  (1)FzY1−FyZ1+WxZ2−(R3+R4)Z3=0  (2)R3+R4=(WxZ2−(FyZ1+FzY1)/Z3  (3)FxY1+(Wx+Wy+Fy)X1/2−(R2+R4)X1=0  (4)R2+R4=(Wx+Wy+Fy)/2+FxY1/X1  (5)R1+R3=(Wx+Wy+Fy)/2−FxY1/X1  (6)Therefore, by inputting numeric values used in the model to these, forces at the respective points are obtained, and by adjusting these to the specified points of a linear guide, a displacement forming an error can be found.
The guide used for the calculations is an LM guide HSR45L made by THK Co., Ltd. In this case, when, supposing that Fy=Fy=5 kN, Wx=6 kN, Wy=2 kN, calculations are approximately made, the error in a width change of R becomes 17 kN, and the displacement in a deformed data in the Y-direction of the guide derived from this becomes about 25 μm, and this amount is not ignorable. It is not impossible to carry out a correction in this direction by using a two-dimensional scale; however, since the measurement needs to be carried out, with the scale being made to stand in the Y-direction, it is not carried out easily. In order to carry out the correction in this direction, a scale is required by which three-dimensional measurements can be carried out.
On the other hand, in the case when displacement fluctuations due to own weight and force of inertia are observed in the same manner, from equilibrium condition among moments of inertia when the Y-axis is shifted, the following equations are satisfied in the same manner:WxZ2=(R3+R4)Z3  (7)(R1+R2)Z3−(Wy±Wy)Z3−Wx(Z3−Z2)=0  (8)Moreover, from equilibrium condition among moments of inertia when the X-axis is shifted, the following equations are satisfied.WyY1+WxY2+(Wx+Wy)X1/2=(R2+R4)X1  (9)−WyY1−WxY2+(Wx+Wy)X1/2=(R1+R3)X1  (10)
In the case of calculating R supposing that the acceleration is 1G by adding changes of R in the case of individual movements, it is changed with a width of 8.8 kN, and it is found that the displacement in the Y-direction due to this change is about 10 μm, which is a value that has to be put into consideration in high-precision machining. Moreover, errors in the X and Z directions due to own weight and moments of inertia can be removed at the initial correction in this structure; however, errors due to changes with time, such as wearing, cannot be removed.
When examinations are made as to what degree the initial correction value is changed to by an expected change in machining conditions, such as changes in moments due to a deflection of a bed caused by a weight change of a workpiece and a change in the center of gravity, it is found that in order to ensure machining precision, a monitoring process can be carried out to find that the change in the correction value is small relative to desired precision or an additional correction can be carried out in accordance with the change in conditions. Of course, these corrections are not required when a two-dimensional or a three-dimensional scale serving as reference is used for controlling or when a monitoring scale is always used in addition to a controlling scale.
However, a special scale of a two-dimensional or three-dimensional type has a disadvantage in that a large detection area is required. For this reason, always mounting this scale onto a machine results in a bulky machine size, and since the scale of this type requires a detection area that is larger than that of a general-use one-dimensional scale by two digits or more, manufacturing costs become very high, with the result that it becomes difficult to provide a high cost performance although high precision is achieved. Moreover, from the viewpoint of reliability, since a difficult designing process is required to install a protector for protecting the wide detection area from chips and coolants, and since even in the case of using a cover the same as that of the one-dimensional scale, a trouble occurrence rate becomes a multiple of the detection area, it is not a practical solution to always use a two-dimensional or three-dimensional scale for controlling or correcting a machine tool.
Moreover, it has been known that an optical scale device that optically reads scales recorded on an optical scale is inappropriate for use in measurements in poor environments contaminated with cutting fluids, chips and the like, while a magnetic scale device that magnetically reads scales recorded on a magnetic scale is durable to such poor environments.